fix(playback): 播放页/迷你播放器立即显示已有封面，不再等元数据流程

原因有两层：
1) Song 模型没有封面路径，播放时只带 url/title/artist，
   播放页只能等元数据服务返回 artwork 字节后才显示；
2) 元数据回来后用新 Song 覆盖当前歌曲时丢了 artworkPath，
   即使带上了也会被冲掉。

- Song 增加 artworkPath 字段
- PlaybackController / 首页收藏 / playPlaylist 构造 Song 时带上
- 播放页：artwork 字节优先，其次直接读 artwork_path 文件
- 迷你播放器同样加 FileImage 兜底
- 元数据更新重建 Song 时保留 artworkPath

真机验证：点击播放后封面立即出现，不再先显示音符占位。

lib/controllers/playback_controller.dart

@@ -98,6 +98,7 @@ class PlaybackController {
     final dbSong = await _db.getSongByPath(songPath);
     final song = Song(
       id: songPath,
+      artworkPath: dbSong?['artwork_path'] as String?,
       title: dbSong?['title'] as String? ??
           songPath.split('/').last.replaceAll(RegExp(r'\.[^.]*$'), ''),
       artist: dbSong?['artist'] as String? ?? '未知艺术家',
@@ -117,6 +118,7 @@ class PlaybackController {
       if (dbSong != null) {
         queue.add(Song(
           id: item.path,
+          artworkPath: dbSong['artwork_path'] as String?,
           title: dbSong['title'] as String? ??
               item.name.replaceAll(RegExp(r'\.[^.]*$'), ''),
           artist: dbSong['artist'] as String? ?? '未知艺术家',